Blends based on vinyl-aromatic polymers having high tenacity and impact strength

ABSTRACT

Blends based on vinyl-aromatic polymers having high tenacity and impact strength, comprising: 
     a vinyl-aromatic copolymer containing from 2 to 25% by weight of an ethylenically unsaturated nitrile and a rubber amount not higher than 15% by weight; and 
     a grafted polymer consisting of an elastomeric core having a transition temperature of the second order lower than 10° C. containing, in the grafted form, chains of vinyl monomers.

This is a division of application Ser. No. 303,954 filed Jan. 31, 1989now U.S. Pat. No. 4,937,280, which in turn is a continuation of Ser. No.188,074, filed Apr. 28, 1988, now abandonded, which in turn is acontinuation of Ser. No. 889,527, filed July 25, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to blends based on vinyl-aromatic polymershaving high tenacity and impact strength.

More particularly, the present invention relates to blends ofvinyl-aromatic polymers having high tenacity, suitable for producingshaped articles endowed with good impact strength properties.

2. The Prior Art

As known, the vinyl-aromatic polymers in question are prepared bypolymerization, either in mass or mass-suspension, of solutions ofpolybutadiene rubbers or styrene-butadiene rubbers, or saturated rubbersof the ethylene-propylene type, in a styrene-acrylonitrile mixture. Theresulting polymer is suited to the production of molded or extrudedarticles having a fairly good tenacity, which is sufficient for normaluses but insufficient for those applications, in which a high impactstrength is required.

This is due to the fact that by said polymerization procedure (otherthan the polymerization in emulsion generally utilized for theproduction of the ABS resins) it is possible to employ only reducedamounts of rubber, owing to the too high viscosities of thepolymerization mass.

A way to overcome these lacks of properties is that of preparing blendswith other polymers which possess the lacking properties, in order toobtain a material exhibiting the desired combination of properties.However, this approach was successful only in a few cases: generally, infact, the blending results in the combination of the worstcharacteristics of each component, so that a material with so poorproperties is obtained, that it has no commercial or practical value.

The reasons for this drawback depend on the fact that not all thepolymers are compatible with one another, and therefore they do notperfectly adhere. This gives rise to interfaces between the componentsof the mixture, such interfaces representing the weak and break points.

THE PRESENT INVENTION

The Applicant has now found that by mixing a vinyl-aromatic copolymercontaining, in the copolymerized form, from 2 to 25% by weight of anethylenically unsaturated nitrile and a rubber amount not exceeding 15%by weight, with a grafted polymer consisting of an elastomeric basehaving a transition temperature of the second order below 10° C. andcontaining, in the grafted form, chains of vinyl monomers, it ispossible to obtain a material which exhibits a high tenacity and a highimpact strength and in which the components are compatible with oneanother and perfectly adhere to one another.

Thus, object of the present invention are blends comprising:

a) a vinyl-aromatic copolymer containing from 2 to 25% by weight of anethylenically unsaturated nitrile and a rubber amount not higher than15% by weight; and

b) a grafted polymer consisting of an elastomeric core having a secondorder transition temperature lower than 10° C. and containing, in thegrafted form, vinyl monomers chains.

The proportions between the two components a) and b) can be varied overa wide range, although values from 10 to 98%, and preferably from 35 to85% by weight of a), and, correspondingly, from 90 to 2%, and preferablyfrom 65 to 15% by weight, of b) are generally employed.

The term "vinyl-aromatic copolymer", whenever used in the presentdescription and in the claims, means any thermoplastic solid polymer andrelevant copolymer composed for the most part, i.e. containing,chemically bound, at least 50% by weight, of one or more vinyl aromaticcompounds having general formula: ##STR1## in which X is hydrogen or analkyl radical having 1 to 4 carbon atoms; n is zero or an integer from 1to 5 and Y is a halogen or an alkyl radical having 1 to 4 carbon atoms.

Examples of vinyl-aromatic compounds having the above general formulaare: styrene; methyl-styrene; mono-, di-, tri-, tetra andpenta-chloro-styrene and the corresponding alpha-methyl-styrenes,nucleus-alkylated styrenes and the corresponding alpha-methyl-styrenessuch as ortho- and para-methyl-styrenes, ortho- and para-ethyl-styrenes,ortho- and para-methyl-alpha-methyl-styrenes, etc. These monomers areutilizable either alone or in admixture with one another or with othercopolymerizable comonomers such as for example maleic anhydride.

The rubbers are employed to render the vinyl-aromatic polymers impactresistant and, according to the present invention, the rubber contentshall not exceed 15% by weight. Amounts from 2 to 12% by weight arepreferred.

The rubbers utilized to this purpose are: polybutadiene, polyisoprene,the butadiene and/or isoprene copolymers with styrene or with othermonomers, or the saturated rubbers of the ethylene-propylene type, theethylene-propylene-diene terpolymers, silicone rubbers with unsaturatedgroups, and the like.

Ethylenically unsaturated nitrile, copolymerized with the vinyl-aromaticmonomer, means, first of all and preferably, acrylonitrile; otherethylenically unsaturated nitrile monomers such as methacrylonitrile canbe advantageously utilized.

The modified vinyl-aromatic copolymers can be prepared according to anyconventional polymerization process, either in suspension, ormass-suspension or continuous mass, provided the above-mentionedcompounds are utilized as starting monomers.

Said modified vinyl-aromatic copolymers differ from the knownacrylonitrile-butadiene-styrene resins, which are known as ABS resins,as regards morphology, structure and size of the rubber particlesdispersed in the rigid polymeric matrix. As a consequence of suchmorphological differences, the blends of the present invention exhibit aresilience or impact strength exceeding that of the ABS resin,particularly when the rubber content is lower than 15% and preferablylower than 10% by weight.

The grafted polymer utilized to increase the impact strength of thevinyl-aromatic copolymer consists of an elastomeric core containing, inthe grafted form, chains of vinyl monomers.

The elastomeric core having a second order transition temperature lowerthan 10° C. can be polybutadiene, copolymers of butadiene with styreneand/or with acrylonitrile, in which the content of butadiene is higherthan 50% by moles, ethylene-propylene rubbers, ethylene-propylene-dienerubbers (EPDM), or acrylic rubbers. The acrylic rubber can be anelastomer obtained through polymerization of a monomeric systemcomprising: 90 to 99.8% by weight of an alkyl-acrylate, containing from1 to 6 carbon atoms in the alkyl chain; 0.1 to 5% by weight of across-linking monomer and 0.1 to 5% by weight of a grafting agent. Inthe acrylic rubber, the cross-linking agent can be a polyacrylic orpolymethacrylic ester of polyols such as butylene-diacrylate ordimethacrylate, trimethylol-propane-trimethacrylate etc.; among thesemonomers, butylene-diacrylate is the preferred one. In the acrylicrubber the grafting agent can be a polyethylenically unsaturated monomerhaving a plurality of reactive groups, which are polymerizable byaddition, such as those containing the allyl group. Examples of suitablegrafting monomers can be the allyl esters of ethylenically unsaturatedacids, such as allyl acrylate, allyl methacrylate, diallyl maleate,diallyl fumarate, diallyl itaconate etc. The function of the graftingmonomer is that of providing a residual content of unsaturation in theelastomeric step, particularly in the last polymerization steps and, byconsequence, at, or in proximity of, the surfaces of the elastomerparticles.

The grafted vinyl monomers of the elastomeric core can be derivatives ofacrylic acid and of methacrylic acid, vinyl-aromatic compounds,vinyl-cyanide compounds and polyfunctional derivatives, either alone orin admixture with one another. Specific examples of these grafted vinylmonomers comprise alkyl esters of methacrylic acid, in which the alkylradical contains from 1 to 16 carbon atoms, preferablymethylmethacrylate, esters of methacrylic acid with polyfunctionalalcohols such as 1, 3-butylene-glycol dimethacrylate andtrimethylol-propane-trimethacrylate, allyl methacrylate and/or diallylmethacrylate; vinyl-aromatic compounds such as styrene, vinyltoluene,alpha-methyl-styrene, halogenated styrene, vinyl-naphthene ordivinyl-benzene, styrene being particularly preferred; compounds ofvinyl-cyanide such as acrylonitrile, methacrylonitrile,alpha-halogenated acrylonitriles, acrylonitrile being particularlypreferred.

These vinyl monomers can be used either alone or in admixture.

The grafted copolymer utilized in the blends which are the object of thepresent invention has preferably an elastomer content higher than 35%and up to 95% by weight.

These copolymers can be prepared by any known method such as bulkpolymerization, polymerization in suspension, bulk-suspensionpolymerization, polymerization in solution or polymerization inemulsion. In the preparation of the grafted copolymer, a homo- orcopolymer of the vinyl monomer can be directly formed in the reactionproduct, or the reaction product as such can be used as a graftedpolymer.

Preferably, one or more vinyl monomers are grafted to the elastomericcore.

A typical example of grafted polymer to be used in the present blend maybe a three-step polymer having a rubber-like core based on butadiene, asecond step polymerized by styrene and a final step, or shell,polymerized by methyl-methacrylate and1,3-butylene-glycol-dimethacrylate.

Another example of grafted polymer to be used in the present blend canbe a two-step polymer: the first step, consisting for 60-95% by weightof the polymer, obtained starting from a monomeric system comprising95-99.8% by weight of butyl acrylate, 0.1-2.5% by weight ofbutylene-diacrylate and 0.1-2.5% of allyl methacrylate or diallylmaleate, and the final step obtained from a polymerized compoundcontaining from 60 to 100% by weight of methylmethacrylate.

The grafted polymers cited hereinbefore are well known commercialproducts and are available from a great number of manufactures such as,for example, Rohm and Haas Company, Philadelphia, U.S.A., under thetrade-name Acryloid KM 653 and KM 323, or from Kanegafuchi, under thetrake-name KaneACE B 28, etc.

To the blends object of the present invention it is possible to addother polymers compatible therewith, such as polycarbonate, polyesters,thermoplastic polyurethanes, polymethacrylates,styrene-methylmethacrylate copolymers, acrylic polymers, ABS,styrene-maleic anhydride copolymers, SAN and other engineering polymers,vinyl chloride polymers, etc. Such engineering polymers or polymers canbe added in any ratio, such as for example from 1 to 90% by weightreferred to the blend.

Furthermore, the blends of the present invention can be co-extruded withsaid polymers or engineering polymers to provide composites havingspecific characteristics for the individual applications.

The blends forming the object of this invention are preparable by hotmixing, in any known mixing unit, such as single-screw and two-screwextruders, Banbury mixers, mixing rollers and the like, at temperatureranging from 180° to 260° C.

The compositions may contain stabilizers or other intimatelyincorporated additives, such as plasticizers, lubricants, antiflameagents, antistatic agents, dyestuffs, pigments, glass fibres or otherinorganic fillers etc., in order to impart particular characteristics tothe material.

The blends, object of the present invention, are easy to be processedand exhibit a complex of properties which make them suited to beutilized for preparing articles having a high tenacity along with a highimpact strength. Said mixes are therefore used in the field of thehousehold electrical apparatus, in the field of electronics andtechnical articles in general, in the form of films, sheets, strips,tapes, rods, boxes, cups, containers and the like. The blends areutilizable for producing foamed articles, using the conventionaltechniques of the art.

For a better understanding of the present invention and to facilitatethe embodiment thereof, a few illustrative but not limitative examplesare given hereinafter.

In the examples, unless otherwise specified, all parts and percentageare by weight.

EXAMPLES 1-3

By means of a single-screw extruder Bandera TR 45, with alength/diameter ratio of 25, there were extruded, under degassing and ata temperature of 210° c., blends consisting of:

a) a vinyl-aromatic copolymer consisting of 72% by weight of styrene, of12% by weight of alpha-methyl-styrene and of 8% by weight of butadienerubber and of 8% by weight of acrylonitrile, and

b) a grafted polymer of the type indicated in Table 1.

The weight ratios between the two components of the mix are reported inTable 1.

By cutting the strands leaving the extruder, granules were obtained,which were dried for 2-4 hours at 80°-90° C.

To determine the characteristics, the granules were injection molded ata temperature of 210° C. in a press NEGRI & BOSSI V-17-110 FA in orderto obtain samples having dimensions in compliance with the standards.

The properties measured on the samples so obtained are reported in thefollowing Table 1.

For measuring the characteristics of the blends, object of the presentinvention, the following methods were employed:

A. Mechanical Properties

There were measured the tensile strengths and elastic modulus, accordingto ASTM D 638, and IZOD resilience with notch at 23° C., according toASTM D 256, on 3.2 mm thick samples.

B. Thermal Properties

Softening temperature VICAT B (5 kg in oil) was determined according tostandard ISO 306.

C. Rheological Properties

The melt index was determined according to standard ASTM D 1238 at 220°C. and 10 kg.

                  TABLE I                                                         ______________________________________                                                           EXAMPLES                                                   COMPOSITION      UNIT    1       2     3                                      ______________________________________                                        (a) Vinyl-aromatic copolymer                                                                       100     85      85                                       (b) Grafted polymer                                                                         A              15      --                                                     B              --      15                                       A. Mechanical properties                                                      Tensile strength                                                              yield strength   MPa     30      27      28,5                                 tensile stress   MPa     31      29      29,5                                 elongation at break                                                                            %       45      64    69                                     elastic modulus  MPa     1800    1550  1450                                    Resilience IZOD                                                              1/2" × 1/8"                                                                              J/m     90      120   110                                    1/2" × 1/2"                                                                              J/m     80      110   105                                    B. Thermal properties                                                         Vicat B          °C.                                                                            98      93    93                                     C. Rheological properties                                                     Melt index       g/10'   12        5,5  5                                     ______________________________________                                         A = ACRYLOID KM 323 B                                                         B = ACRYLOID KM 653                                                      

EXAMPLES 4-8

By operating under the same conditions as in examples 1 to 3, granuleswere prepared starting from blends composed by:

a) a vinyl-aromatic copolymer consisting of 72% by weight of styrene, of20% by weight of acrylonitrile and of 8% by weight of polybutadienerubber;

b) a grafted polymer of the type reported in Table II, and optionally

c) an aromatic polycarbonate type "SINVET" sold by ANIC, San DonatoMilanese - MILANO.

The weight ratios of the blends are reported in Table II as well as theproperties measured on the samples obtained from said blends.

                                      TABLE II                                    __________________________________________________________________________                       EXAMPLES                                                   COMPOSITION    UNIT                                                                              4   5   6   7   8                                          __________________________________________________________________________    (a) Vinyl-aromatic copolymer                                                                     85  85  85  70   50                                        (b) Grafted polymer                                                                       A      15  --  --  --  --                                                     B      --  15  --  30     7,5                                                 C      --  --  15  --  --                                         (c) Aromatic polycarbonate                                                                       --  --  --  --    42,5                                     A. Mechanical properties                                                      Tensile strength                                                              yield strength MPa 32  27    27,5                                                                            25   41                                        tensile stress MPa 29  29    26,5                                                                            27   44                                        elongation at break                                                                          %   29  61  52  75  102                                        elastic modulus                                                                              MPa 1950                                                                              1600                                                                              1700                                                                              1450                                                                              2000                                        Resilience IZOD                                                              1/2" × 1/8"                                                                            J/m 190 160 115 210 450                                        1/2" × 1/2"                                                                            J/m 145 135 100 185 250                                        B. Thermal properties                                                         Vicat B        °C.                                                                        92  93  93  88  112                                        C. Rheological properties                                                     Melt index     g/10'                                                                               5,5                                                                              5    8,5                                                                               3,5                                                                                8,0                                     __________________________________________________________________________     A = ACRYLOID KM 323 B                                                         B = ACRYLOID KM 653                                                           C = Kane ACE B 28                                                        

We claim:
 1. Blends based on vinyl aromatic polymers comprising:10-99%by weight of a blend consisting of:a) from 10 to 98% by weight of avinyl aromatic copolymer containing from 2 to 25% by weight of anethylenically unsaturated nitrile and a rubber not exceeding 15% byweight, and b) from 90 to 2% by weight of a core-shell polymerconsisting of an elastomeric core having a second order transitiontemperature lower than 10° C. and a shell consisting of vinyl monomersgrafted to the elastomeric core, selected from the group consisting ofderivatives of acrylic acid, derivatives of methacrylic acid, vinylaromatic compounds, vinyl cyanide compounds, polyfunctional derivativesand mixtures thereof, the content of the elastomer being higher than 35%and up to 95% by weight, the blends of the vinyl aromatic polymershaving a high tenacity and impact strength and a melt index from about3.5 to about 8.5 g/10', and from about 1 to 90% by weight of acompatible polymers selected from the group consisting of polycarbonate,polyesters, thermoplastic polyurethanes, polymethacrylates,styrene-methylmethacrylate copolymers, acrylonitrile-butadiene styrene,styrene-maleic anhydride copolymers, styrene-acrylonitrile copolymers,and vinyl chloride polymers.
 2. The blends according claim 1, whereinthe elastomeric core of the grafted polymer, having a second ordertransition temperature below 10° C., is selected from the groupconsisting of polybutadiene, copolymers of butadiene with styrene andwith acrylonitrile in which the butadiene content is higher than 50% bymoles, ethylene-propylene rubbers, ethylene-propylene-diene rubbers andacrylic rubbers.
 3. The blends according to claim 1, wherein thecore-shell polymer is a three-step polymer having a rubber core based onbutadiene, a second step polymerized from styrene and a final step, orshell, polymerized from methyl-methacrylate and1,3-butylene-glycol-dimethyacrylate.
 4. The blends according to claim 1,wherein the core-shell polymer is a two-step polymer: the first step,consisting of 60 to 95% by weight of the polymer, obtained starting froma monomeric system comprising 95-99.8% by weight of butyl acrylate,0.1-2.5% weight of butylene-diacrylate, and 0.1-2.5% of allylmethacrylate or diallyl maleate, and the final step obtained from apolymerized compound containing from 60 to 100% by weight ofmethyl-methacrylate.